There have been many threads on CN about exactly when does a Paracorr become a necessity. Most posters have felt that this occurs at f/4.5 or faster. They also have indicated that at f/4.9 a coma corrector was nice to have but that it was not a necessity. This has made me reluctant to spend $330 for a Paracorr for my Orion XT12i f/4.9 scope.

The new Astro-Tech coma corrector at the lower price of $135 renewed my interest in a coma corrector. I purchased one from Astronomics a month ago.

The instructions that come with the ATCC state that the optimum spacing from the lens assembly/shoulder to the image plane of the EP is 73.5mm. The distance from the top of the 2" eyepiece holder of the unit is 26.5mm to the image plane of the EP. I used this 26.5mm length as my goal for spacers.

The lack of an adjustable top for the ATCC limits its usefulness. You can add spacers between the bottom lens assembly and the top EP holder of the ATCC, or you can add spacers to the bottom of each eyepiece. I chose to add spacers between the lens assembly and the EP holder.

The two parts of the ATCC are connected by standard 2"/48mm filter threads. The 14mm and 28mm Baadder Hyperion fine tuning rings and a 2" color filter were picked up from OPT. Removing the glass from the filter left me with a 6mm tuning ring.

For 1.25" EP I use the 14mm and 6mm ring between the 2 parts of the ATCC. The 2"/1.25" adapter in the top of the EP holder adds another 6mm for a total of 26mm. For 2" EP I use the 28mm ring.

This 'one size fits all and use it like a barlow' approach works fairly well. Visually judging the amount of coma correction and field flattening from one combination to another is harder than I thought it would be. My subjective and unscientific estimation is that for most of the eyepieces in my collection about 1/2 to 2/3 of the edge coma effects are removed. The area of the flattest portion of the FOV increases as well.

Improvement varies from one EP design to another. Premium EP most clearly showed improvement. The exact position of the focal plane of each EP also has an effect.

Now for the big question: Is it worth it? I have mixed feelings about the ATCC. I think it is great for wide field low to medium magnification views. It needs more testing on a night of good seeing before I will decide to use it for high power viewing. The bottom line is that at f/4.9 the ATCC is nice to have, but it is not a necessity. For the relatively low price of the ATCC compared to a Paracorr I am happy with it.

Random thoughts in no particular order:

This is not intended as a scientific report. Someone else will have to do that. To put it another way, "**** Jim! I'm a Pharmacist, not an Optician!"

This thing NEEDS an adjustable top. Are you listening Atro-Tech?

You will need at least 8 to 12mm of in focus (lower focuser tube top). This distance will vary with different spacers and different EP. I ran out of in focus several times with various combinations.

Perhaps you could add one of these to it. http://www.televue.c...emDesc=Tunable%
It's an addon turntable built for the origional Paracorr. Probably by then you will be up to the cost of just buying a Paracorr in the first place. I think this show that you often get what you pay for.

Perhaps you could add one of these to it. http://www.televue.c...emDesc=Tunable%
It's an addon turntable built for the origional Paracorr. Probably by then you will be up to the cost of just buying a Paracorr in the first place. I think this show that you often get what you pay for.

Bill

Just happens that I have a Para and ATCC in my hot little hands right now just waiting for the sky to clear.

It so happens that the tunable top from the Para does indeed fit onto the ATCC. Problem is that the screw is a different thread on the Para and the screw on the AT body just above the threads is too big. I could see the screw thing being worked out by an industrious soul though.

I have used my ATCC in the 12" f5 Lightbridge once during a short break in the clouds. The 22 Pan was used and the stars were sharp across the field.

The two parts of the ATCC are connected by standard 2"/48mm filter threads. The 14mm and 28mm Baadder Hyperion fine tuning rings and a 2" color filter were picked up from OPT. Removing the glass from the filter left me with a 6mm tuning ring.

Nice report Tom.
Also thanks for mentioning the Baader FT rings fit. As noted in my ATCC thread, there are certain brands that won't thread on or only one end. I'll have to pick up a few for my personal use along with the adjustable top I made.

I have a paracorr and a Baader MPCC, and am still thinking about getting the AT coma corrector. Sky Captain makes a nice top for it. But even at F4.5 I'm finding it hard to justify another coma corrector. Two I can, because on is in Mi. and the other in Ok., but 3??? Only because it might make a better CC than the Baader for visual.

The Baader rings do fit, but the male threads on the rings are a tight fit into the female threads on the ATCC EP holder top. I get about 2 turns before it stops. This does feel safe to use though. Continued use is improving this. The Baader rings are smooth, and it is difficult to get a firm enough grip to unscrew them if they are on too tight.

The filter ring without the filter is flexible under hand pressure and is tricky to thread on. It is also easier to get stuck and harder to get off. Its male threads are also tight on the ATCC top. It fits the Baader rings and the ATCC bottom better than the ATCC top. It is a close out Antares 2" color filter from OPT. I have since found that Surplus Shed has empty 2" filter rings (M2801D) for $6.50. How well these fit is an unknown.

If you decide to make an adjustable top available to others please let me know. When I ordered the ATCC I asked if AT was planning to make an adjustable top. The salesman said "there has been some talk" about it. What this means is hard to say.

Thanks also for the info on the difficulty of using the Paracorr top.

The Orion helical focuser plus the 14mm ring shows some promise as an adjustable top for 1.25" EP. The main problem is that my scope focuser runs out of in focus quickly with this arrangement. Moving the mirror forward would cause problems with some 2" EP that require almost all of the focuser's out travel. Even with an extension tube I am reluctant to make a major forward movement of the mirror. The Orion mirror cell design has the mirror cell attached to the end ring, not to the optical tube wall.

The Baader rings do fit, but the male threads on the rings are a tight fit into the female threads on the ATCC EP holder top. I get about 2 turns before it stops. This does feel safe to use though.

Good to know Tom, gotta find one that screws on all the way. Would be nice if AT would make some accy's that fit instead of the hit an miss that we have been going through. I still can't tell if its the threads on the CC or the tubes that's the problem.

The two parts of the ATCC fit together just fine. I think the problem Kerry is referring to is the mismatch of the 'standard' screw threads on 2" accessories from one supplier to the next. You never know for sure if a new 2" filter or accessory will fit your other 2" items until you try fitting them together. If Astro-Tech would make some tuning rings or an adjustable top for the ATCC this problem would be unlikely to occur with all AT parts.

Sometimes the problem is too much paint in the threads and all you need to do is screw and unscrew the two items until the paint wears off. Sometimes it involves the metal not being machined to standard thread dimensions. I have an Orion 2" moon filter (marked Korea) that would not fit an Orion 40mm Optiluxe 2" EP. I had to use a triangular needle file to open up the male threads on the filter before it would fit.

In my case the best fit is: ATCC lens assembly to Antares filter ring to Baader ring to ATCC EP holder. This arrangement has all the screw joints fully engaged except for the Baader ring to EP holder joint which is on only ~2 revolutions.

I have been trying to improve the performance of the ATCC in my scope. Checking previous coma corrector threads on CN produced an interesting item: Someone stated that a coma corrector helped to reduce the coma to a point to where your eye's own accommodation of focus would help to obtain a sharp focus at the edge of the FOV.

Uh-oh. I am very near sighted and wear trifocals. The only good things about my eyesight is that there is no astigmatism or floaters. I always flip my glasses up onto my forehead to look through an EP.

I tried viewing with a set of single vision eyeglasses that I use for binoculars with my long eye relief 2" eyepieces. Bingo! Edge correction through the ATCC improved to the proverbial "pin point stars from edge to edge" level.

My new plan is to use the ATCC with 2" EP and glasses for low power wide field views. 1.25" EP will continued to be used "bare eyed" with the ATCC since few of the 1.25" EP in my collection have eye relief long enough for use with glasses.

Excellent thread. I am about ready to pull the trigger on this but may just wait for a little bit more. My 14" f4.6 may need it (haven't had it out yet) and not sure on the AT or the paracorr (it's the price thing that stops me on the paracorr).

I pulled the trigger on the AT Coma Corrector last Tuesday. I haven't received a shipping confirmation e-mail. The coma corrector is on back-order. Well, it is an early Christmas present for myself, but I hope it arrives well before Christmas. Halloween would be nice.

I want to use it with my ES 14 100 Degree eyepiece for wide-field views of DSO with my 10" f/4.8 Newt Dob. But I would also like to try it with my binoviewer, ideally so I can have a wide flat coma-free field for observing planets without tracking.

I want to use it with my ES 14 100 Degree eyepiece for wide-field views of DSO with my 10" f/4.8 Newt Dob. But I would also like to try it with my binoviewer, ideally so I can have a wide flat coma-free field for observing planets without tracking.

Mike

I am not sure how well coma correctors work with binoviewers. The spacing between the focal plane of the eyepiece and the coma corrector is important/critical in optimizing the correction of coma and it seems to be a matter of a few millimeters. The MPCC with it's spacing needs and the Paracorr with it's included tunable top clearly demonstrate this.

But with a Binoviewer, the distance from the coma corrector to the focal plane of the eyepiece is several inches, this seems like a real problem. Ideally, you could have a coma corrector for each eyepiece but obviously that is impractical.

That does make sense^ ill put jupiter at the edge of my fov and watch it drift through the ep but it only looks good toward the center anyway....

That would depend on how well-corrected the eyepiece is, also. But in general, with the coma corrector the planet's image should be sharper beyond the usual diffraction delimited area for the telescope.

I am not sure how well coma correctors work with binoviewers. The spacing between the focal plane of the eyepiece and the coma corrector is important/critical in optimizing the correction of coma and it seems to be a matter of a few millimeters. The MPCC with it's spacing needs and the Paracorr with it's included tunable top clearly demonstrate this.

But with a Binoviewer, the distance from the coma corrector to the focal plane of the eyepiece is several inches, this seems like a real problem. Ideally, you could have a coma corrector for each eyepiece but obviously that is impractical.

Something to think about before diving in...

Well, I've already dived in, because I ordered the AT Coma Corrector last week. But it's on backorder, so I still have a chance to come out of the water.

My intended use of the ATCC is not solely dependent on binoviewing. I'm flexible. If binoviewing is not possible with the corrector, I'll go back to cyclops mode. I'll just try some of my wide-field EPs, maybe with a Barlow or lens cell screwed on for higher power. Binoviewing planets is great, but monoviewing with a wide flat field in a non-tracking Newt should also be very nice.

But I will definitely try the corrector with my binoviewer to see if I can get it to work. Of course, dual CCs is out of the question for several reasons: (1) cost, (2) weight, (3) my binoviewer is not a 2".